Low sulfur recarburizer coke is a type of coke used in the production of high quality steels. Its purpose is to increase the carbon content of the steel without introducing any extraneous contaminants, especially sulfur and nitrogen. Historically, steel producers and recarburizer marketers have used crushed scrap graphite (graphitized premium coke) as the major source of recarburizer coke. However, this source has steadily declined as scrap rates in the graphite electrode production, and electric arc furnaces have been reduced. A market now exists for alternative sources of recarburizer coke with very low levels of contaminants.
It would be possible, of course, to manufacture high quality, premium coke, calcine and graphitize this material and use it as recarburizer coke. However, such premium coke is too valuable in its use for electrodes in the manufacture of steel and, it would not be profitable to use this material as recarburizer coke. Prior to graphitization, premium coke usually contains substantial amounts of sulfur and nitrogen, up to 0.3 to 0.5 or higher weight percent sulfur and nitrogen in similar quantities. Thus, ungraphitized premium coke would not be suitable for use as recarburizer coke even if economics would permit its use. Another type of coke which is manufactured in substantial quantities is so called aluminum grade coke, that is, coke which is used in manufacturing electrodes for use in the production of aluminum. This coke also contains substantial amounts of sulfur and nitrogen which make it unsuitable for use as recarburizer coke.
It has been found that FCC decant oil (also known as slurry oil or clarified oil) can be processed to produce recarburizer coke. In order to use decant oil for this purpose it must first be subjected to catalytic hydrotreating to reduce its sulfur and nitrogen content. Unfortunately the severe hydrotreating conditions which are required to produce a feed material of reduced sulfur and nitrogen content, suitable for making recarburizer coke, rapidly deactivate the hydrotreating catalyst. This results in a major decrease in catalyst life and increasing cost of the operation.
In accordance with this invention, FCC decant oil is subjected to vacuum distillation to separate it into two fractions, a vacuum gas oil in which sulfur and nitrogen are concentrated and a heavy residuum containing materials which tend to coke under severe hydrotreating conditions. The vacuum gas oil is catalytically hydrotreated under severe conditions to reduce the sulfur and nitrogen content to low levels; the hydrotreated product is then thermally cracked to provide a thermal tar which is subjected to delayed coking and the delayed coke is calcined to provide a recarburizer coke product containing not more than 0.1 weight percent sulfur and not more than 0.1 weight percent nitrogen.